1. Technical Field
The present invention relates generally to transformers and, in particular, to electronically controlled transformers used in AC or DC arc welding power supplies or other applications where it is desirable to control the output of a transformer.
2. Background Art
The prior art is replete with methods and devices to control the output of a transformer. Some such devices include the use of switches, such as thyristors, to control the phase of either the primary or secondary current, thereby controlling the transformer output. These devices offer a large control range and typically low power consumption by the control circuit. However, the on/off nature of the control devices drastically disturbs the output waveform. This adversely affects the performance of devices such as welding machines making such devices useful only for specific applications. Moreover, when such a transformer is used in a welder or other high current application, the control circuitry must be capable of handling high current levels, thereby increasing the cost of the equipment.
Other devices utilize a magnetic core as a shunt in the magnetic circuit to decouple the primary and secondary windings and thus control the output voltage of the transformer. Devices such as these control the flux shunted through the shunt core, thereby controlling the flux through the secondary core and the output of the transformer.
The flux shunted through the shunt core may be controlled by physically moving the shunt core in and out of the magnetic circuit. However, such a mechanical control is not well suited for use with a remote control. Moreover, the forces on a shunt core are sufficient to cause a movable shunt core to vibrate and may create undesirably loud noise.
Another example of a shunt controlled transformer is shown in U.S. Pat. No. 4,177,418 issued to Breuckner et al Dec. 4, 1979. Breuckner discloses a transformer having a two-legged shunt core and a coil wrapped around each leg. The shunt coils are electrically connected in series, but with a reversed polarity, causing the AC current induced in the shunt coils to be in opposite directions and cancel. A switch in series with the shunt coils is opened and closed, selectively allowing DC current to flow through the shunt coils, thereby maintaining the output of the transformer within a predetermined range of a desired level.
The switch in the Breuckner arrangement is part of a control circuit having an independent source of DC power. Moreover, the shunt coils, primary coil, and secondary coil are disclosed as being disposed parallel to one another, thereby increasing the size of the transformer. The shunt coils are also positioned in a plane other than the plane of the primary and secondary coils, further increasing the size of transformer.
Presently known control systems are unsatisfactory in several regards. The waveform of the output of the transformer is often undesirable because the output current goes to zero when the control system is regulating the output. Many require a separate reactor which is expensive, some require a physical construction that is undesirably large, others require expensive high current control components. Accordingly, the need exists for a simplified yet economically efficient electronically controlled transformer.